It is sometimes desired to provide vehicular rubber tires with reduced rolling resistance to promote fuel economy for the associated vehicle.
The use of silica reinforcement together with a silica coupling agent in a tread rubber composition has been mentioned for promoting improved (reduced) rolling resistance for a tire where a solution polymerization prepared styrene/butadiene rubber (S-SBR) is used (for example see U.S. Pat. No. 5,227,425) and where a functionalized styrene/butadiene rubber is used (for example see U.S. Pat. Nos. 7,202,306 and 7,222,650).
Rubber compositions with reduced reinforcing filler contents (for example reduced carbon black contents), rubber compositions with increased crosslinking density and rubber compositions which contain one or more elastomers with a relatively low glass transition temperature (Tg) have been proposed for tire treads to promote a reduction in the rubber hysteresis of the tread and an associated lower rolling resistance for the tire itself.
However, it is well known to those skilled in such art that reducing the tire tread rubber hysteresis for a purpose of reducing the rolling resistance of the tire typically results in a reduction of one or more other desirable tire tread properties such as, for example, a reduction in tread traction, or tread grip.
It is therefore desirable to promote a reduction in hysteresis of a rubber composition for a tire component, usually predictive of an increased rebound property of the rubber composition itself, to thereby promote a reduction in a tire's rolling resistance without significantly degrading one or more various other tire performance properties.
Such contemplated tire components for this invention are a tire sidewall, particularly an outer tire sidewall rubber layer which does not contain cord reinforcement and is exposed to atmosphere conditions, physical scuffing and substantial flexing, as well as a tire undertread (which might be referred to as a subtread) rubber layer which also does not contain cord reinforcement and which underlies the tire outer tread rubber layer with its running surface for the tire tread. Such undertread rubber layer may be found in a tire with a tread of a cap/undertread construction with an outer tread cap rubber layer which contains the running surface of the tire and an undertread rubber layer as a tread base layer or an intermediate rubber layer positioned intermediately between a tread outer rubber cap layer and a tread base rubber layer.
A significant aspect of this invention is a focus on a tire's undertread (subtread) rubber layer in a sense that the rubber composition used for the undertread rubber layer may be used to promote a reduced rolling resistance for the tire in the sense of the rubber composition having a higher rebound value, without significantly compromising various other physical properties of the tread, and to thereby promote an increased fuel efficiency for an associated vehicle.
An additional significant aspect of this invention is a focus upon an outer tire sidewall rubber layer which also contributes to a tire's rolling resistance because of its relatively large deformation (e.g. considerable flexing) during working of the tire where hysteresis loss of its rubber composition and associated tendency of increased internal heat generation may be a factor as described in a paper entitled “Fundamentals of Rolling Resistance” published in Rubber Chemistry and Technology, Volume 74(3), Pages 525 through 539 (year 2001). This is considered significant for use of the rubber composition as an outer tire sidewall rubber layer in a sense of the rubber composition demonstrating a higher rebound value and therefore promoting a prospectively lower internal heat generation and lower hysteresis loss and thereby promoting an increased fuel efficiency for an associated vehicle.
A unique advantage of promoting a reduction in rolling resistance of tires through the modification of the rubber composition of the outer tire sidewall component, whether by material variations or by structural changes to promote a reduced hysteresis loss of the sidewall rubber composition is that tire tread tire performances, such as treadwear and traction, are less likely to be compromised.
In practice, an outer tire sidewall component is often comprised of a mixture of natural rubber and polybutadiene rubber. Particulate rubber reinforcement in a form of carbon black is historically used in the rubber composition for the outer sidewall rubber layer.
In some instances, an inclusion of silica together with carbon black has been suggested for tire sidewall rubber compositions. For example, see U.S. Pat. No. 5,244,028 in which a small quantity of silica in an amount of about 1.0 to 5.0 phr, with low pH values to promote resistance to weathering, and U.S. Pat. No. 6,046,266 with about 20 to 60 phr of silica, together with natural rubber and polybutadiene rubber, to promote an increase in durability and cut growth resistance for the rubber composition.
It has now been discovered that hysteresis loss of a rubber composition can be significantly reduced through the use of a functionalized polybutadiene rubber in combination with silica and a silica coupling agent which may be suitable for use as a tire component such as for a tire subtread or tire outer sidewall component, to promote reduced rolling resistance of the tire without significantly degrading one or more other performance factors for the tire.
The term “subtread” relates to a portion of a vehicular tire tread of a multilayered configuration with an outer circumferential tread cap rubber layer with a running surface for ground-contacting purposes and an underlying rubber tread layer which underlies the tread cap rubber layer and is referred to herein as a “subtread” or “undertread”. It is usually not intended to be ground-contacting. For some tread configurations, the subtread is considered as being a tread base rubber layer which interfaces with an underlying overlay layer or tread belt layer. In other tread configurations, the subtread may be an intermediate tread rubber layer positioned between the outer tread cap rubber layer and a tread base rubber layer.
The term “sidewall” component refers to an outer tire sidewall rubber layer normally positioned outside of (axially outward from) the tire carcass ply layers and usually an outer sidewall rubber layer which may be visually observable and atmospherically exposed. Such sidewall rubber layer is usually subject to extensive flexing scuffing as would be well known to one having skill in such art.
In the description of this invention, the term “phr” where used relates to parts by weight of an ingredient per 100 parts by weight of rubber, unless otherwise indicated.
The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” are used interchangeably unless otherwise indicated. The terms “compound” and “rubber composition” may be used interchangeably unless indicated.